Before transplanting you will need to harden off your seedlings. You can do this by moving them outside in the sun for an hour the first day, 2 the second and so on. This will avoid sun scald on your plants because the sun is so much stronger than grow lights and adjust them to the change in temps and wind. I would start hardening off when temps are above freezing. Things like tomatoes, peppers, and eggplant are not cold hardy so I usually wait till night time temps won't be below 50. Brassicas can usually handle a little bit of a frost. Its a good idea to know your last frost date for your area. When it gets close watch the 10 day forecast make sure it won't go below freezing this is usually a good time to start. If you get a sudden cold snap and already transplanted them you can try covering them with something like harvest guard until its passed.